Transfer or reproduction of an electrostatic charge image is generally conducted in such a manner that a photoconductive layer, which is stacked on an electrode, is fully charged by corona charging in the dark and then exposed to intense light to thereby turn the exposed areas of the photoconductive layer electrically conductive, and the charge in the exposed areas is removed by leaking, thereby optically forming an electrostatic charge image on the surface of the photoconductive layer, and thereafter toner that has electric charge which is opposite in polarity to (or the same as) the residual charge is attached thereto, thereby developing the electrostatic charge image.
This electrophotographic technique cannot generally be used for photographing because of low sensitivity, and it is common practice to carry out toner development immediately after the formation of an electrostatic latent image because the electrostatic charge retaining time is short.
In the meantime, an image recording method by exposure under voltage application has been developed in which a photosensitive member that comprises a photoconductive layer stacked on an electrode is disposed face-to-face with an electric charge retaining medium that comprises an insulating layer stacked on an electrode, and in this state, image exposure is effected with a voltage being applied between the two electrodes, thereby recording an electrostatic charge image of extremely high resolution on the electric charge retaining medium and also enabling the electrostatic charge image retaining time to be lengthened extremely. To transfer such an electrostatic charge image by the toner development as in the conventional practice, image exposure must be effected for each transfer process and the operation is therefore troublesome. Since the electric charge retaining medium has an extremely long electric charge retaining time, the medium itself can be utilized as an information medium, and it has been demanded to enable the electrostatic charge information on the electric charge retaining medium to be directly transferred or reproduced.
There is another known developing method wherein a thermoplastic resin layer having an electrostatic charge image formed thereon is heated to form a dimple pattern image and then cooled to fix the image, thereby developing the electrostatic charge pattern.
According to this developing method, a photoconductive member 10, which comprises an electrode 10b and a thermoplastic resin layer 10a that are formed on a substrate 10c, is uniformly charged by corona charging with a charger 11, as shown exemplarily in FIG. 1(a). Then, image exposure is effected to form an electrostatic charge pattern in the shape of the image, as shown in FIG. 1(b). Thereafter, the photoconductive member is heated with a heater 12, with the electrode 10b grounded, as shown in FIG. 1(c). In consequence, the thermoplastic resin layer 10a is plasticized, and the electric surface charge and the electric charge of the opposite sign that is induced on the electrode 10b in correspondence to the electrostatic charge pattern attract each other. As a result, a dimple pattern image 10a, that is, a frost image, is formed on the surface of the thermoplastic resin layer, as shown in FIG. 1(d). After the formation of the frost image, the photoconductive member is cooled to fix the dimple pattern image, thus enabling development of the electrostatic charge pattern.
However, the conventional developing method shown in FIG. 1 is inferior in the electric charge retaining performance because the electrostatic latent image is formed on the photoconductive member. For this reason, a method has been proposed wherein an electrostatic charge pattern is formed on an electric charge retaining medium which has a thermoplastic resin layer of high insulation quality, to thereby form a frost image. With this method, however, it is impossible to transfer a particular electrostatic charge image many times because the electrostatic charge leaks each time a frost image is formed by heating.
It is an object of the present invention to enable electrostatic charge information formed on an electric charge retaining medium to be transferred to or reproduced on another electric charge retaining medium many times without performing toner development.
It is another object of the present invention to provide an electrostatic charge information reproducing method which enables an electric charge retaining medium to be even more utilized as an information medium.